1. Field of the Invention
The present invention is directed to a sensor including an electrostrictive transducer and, more particularly, to a fiber optic interferometer utilizing an electrostrictive transducer for sensing low frequency electric fields, resolving extremely small voltages at frequencies between dc and 100 Hz (Hertz) when used in conjunction with a high frequency carrier, and for detecting high frequency signals in the range from 100 Hz to 100 kHz with an applied dc bias.
2. Description of the Related Art
Prior art interferometers used as electric field or voltage sensors usually employ piezoelectric materials in a transducing element. The strain e in piezoelectric materials (the strain being the length change of the material divided by the total length of the material) is linearly related to the applied electric field E (e=kE, where k is a constant and E=V/d where V is the voltage and d is the material thickness). When an optical fiber is attached to such a material, the induced strain in the material is transferred to the optical fiber, producing an optical phase shift shift proportional to the strain in the fiber. In measuring low frequency electric fields, however, the low frequency signals cannot be separated from noise or are separated with much difficulty and low accuracy. In addition, the noise spectrum generally exhibits 1/f dependencies which impedes dc and low frequency resolution. Reported values for a minimum detected field for a fiber optic electric field sensor, using a piezoelectric sensing element and employing 60 cm of optic fiber bonded to the piezoelectric material, are in the range of 50-90 V/m/.sqroot.Hz at 10 Hz. Piezoelectric sensing elements are not particularly accurate below 10 Hz because of the noise that interferes with low level signals of interest.
Non-linear transducers such as magnetostrictive alloys (e.g., Metglas 2605S-2) have been employed in fiber optic interferometers to overcome the 1/f noise problem by mixing a signal at a frequency .OMEGA. with a carrier or "dither" at a much higher frequency .omega.. This "up converts" or heterodynes the low frequency signal away from the noise to a region (.omega.+.OMEGA.) where 1/f noise is insignificant. Resolutions for magnetostrictive alloys have a minimum detected field of 10 pT/.sqroot.Hz at 1 Hz. The magnetostrictive alloys, however, are used to detect magnetic fields and not low frequency electric fields or voltages.